The relative motion of a wind powered vehicle with regards to the wind is often referred to as sailing upwind, or sailing downwind. Sailing downwind is accomplished by positioning sails to create as much drag as possible. The resulting wind pressure on the sails “pushes” the vehicle along in the direction of the wind. Essentially any shape that effectively creates drag can be used for downwind sailing. However, when sailing upwind or into the wind, sail shape becomes much more important. In order to sail upwind, the sails are positioned to generate aerodynamic lift in relation to the apparent wind. It is this lift that is translated into the driving force that propels the vehicle forward. When sailing upwind, drag is no longer beneficial, as it counteracts the resulting drive force. Therefore, it is important to developing sails that maximize lift while minimizing drag for sailing into the wind.
Those within the sailing community continue to improve upon existing techniques and create new designs to improve the performance of sails. Certain unconventional designs have an airfoil shape similar to the shape of an airplane wing. The same aerodynamic principles which apply to horizontal aircraft wings also apply, when the wing is placed on end in the vertical axis. When used by wind powered vehicles such as sailboats, these vertical airfoils are often referred to as wing sails.
Wing sails differ from conventional sails in that they have two surfaces of curvature rather than a single thin surface. These two surfaces create a pressure differential by forcing air to flow past them at different velocities, thus creating lift. As a result, high lift airfoils are asymmetrical and will only generate lift efficiently in one direction. This limitation presents a challenge when sailing, where the sail airfoil camber is required to reverse in order to tack. Conventional sails are able to reverse due to the flexibility of their material. However, tacking in this manner is more difficult with a wing sail due to its thick three dimensional shape.
In order to address this tacking issue, numerous wing sails with unique configurations have been developed. One general category is that of a semi rigid or soft wing sail consisting of a flexible covering supported by internal ribs or battens.
An example in the prior art of a soft wing sail capable of tacking is Orrison's Air Foil with Reversible Camber, U.S. Pat. No. 4,341,176. This document discloses an airfoil having a configuration of a plurality of rigid spars, moveable bars, and flexible slats, all covered by a flexible and moveable skin. This airfoil is designed to provide lift which is automatically reversed by changing the angle of incidence of the wind upon the airfoil, resulting in lift in the opposite direction. This design develops an asymmetrical airfoil, while still allowing for efficient tacking by reversing the camber from one side of the sail to the other. One of the limitations of this device is that the airfoil camber is automatically adjusted by the angle of the incident wind. As such, it does not provide a mechanism for manually adjusting the airfoil camber. Another limitation is that it does not provide a neutral symmetrical orientation between the asymmetrical orientations.
U.S. Pat. No. 4,757,779 discloses a reversible airfoil having manual control of the camber of the sail. The disadvantages of this sail are that the airfoil control lines are external to the sail structure, and that the leading edge of the sail requires inflating. The external lines could add drag, may potentially get tangled, and prevent the sail from rotating 360 degrees. The inflatable balloon structure in the leading edge may also interfere with raising and lowering of the sail. It is also at risk for punctures, leading to deflation of the leading edge.
U.S. Pat. No. 4,386,574 describes a sail assembly that is reversible, has a variable profile, and is collapsible. The invention provides a means for control of the airfoil profile independent of the wind. A limitation of this sail assembly is the complex mechanical system of cams, gears, and motors required to adjust the airfoil profile.
U.S. Pat. No. 4,624,203 discloses a batten structure for a soft wing sail. This batten structure allows for both manual and automatic control of the airfoil camber. A limitation of this structure is the number of parts required for the batten structure.
U.S. Pat. No. 5,271,349, also discloses a soft wing sail structure where the airfoil shape is automatically controlled by the wind due to the frequent changes in the force of the wind. With this invention, airfoil shape is limited to adjustments of the tail of the airfoil ribs. It also has a number of linkage mechanisms that add weight and complexity. One of the concerns of automatic control is that the airfoil camber could automatically reverse in turbulent airflow. Automatic control may also be ineffective during periods of strong gusting or light winds.
It is the object of the present invention to overcome the noted disadvantages of the prior art.